Refer him to the Government's own web pages on the subject:
http://www.direct.gov.uk/en/Motoring...le/DG_10014246

Specifically, the bit that says "If a second-hand chassis or monocoque bodyshell is used, the vehicle must pass a an enhanced single vehicle approval (ESVA) or single vehicle approval (SVA) test after which a "Q" prefix registration number will be allocated."

And the wee flap in the carpet being in the wrong place is always going to be a giveaway to any prospective buyer.